Supercapacitors are electrical energy storage systems which are particularly advantageous for applications requiring the conveyance of high-power electrical energy. Their ability to rapidly charge and discharge and the increased lifetime compared with a high-power battery make them promising candidates for a number of applications. Supercapacitors generally consist of the combination of two conductive porous electrodes having a high specific surface area, which are immersed in an ionic electrolyte and separated by an insulating membrane known as a “separator”, which allows ionic conductivity and prevents electrical contact between the electrodes. Each electrode is in contact with a metal current collector, making possible exchange of the electric current with an external system. Under the influence of a potential difference applied between the two electrodes, the ions present within an electrolyte are attracted by the electrode surface exhibiting an opposite charge, thus forming an electrochemical double layer at the interface of each electrode. The electrical energy is thus stored electrostatically by charge separation. The expression of the capacitance C of a supercapacitor is identical to that of a conventional capacitor, namely:
C=ε·S/t, where ε denotes the permittivity of the medium, S the surface area occupied by the double layer, and t the thickness of the double layer.
The capacitances achievable within supercapacitors are nevertheless much higher than those commonly achieved by conventional capacitors, as a result of the use of carbon-based electrodes with a maximized specific surface area and of the extreme thinness of the electrochemical double layer (typically a few nanometers thick). These carbon-based electrodes must necessarily be conductive in order to provide transportation of the electric charges, porous in order to provide transportation of the ionic charges and the formation of the electrical double layer over a large surface area, and chemically inert in order to prevent any energy-consuming parasitic reactions.
The energy E stored within a supercapacitor is defined according to the conventional expression for capacitors, i.e.:E=½·C·V2, where V is the potential of the supercapacitance.
The capacitance and the potential are therefore two essential parameters which it is necessary to optimize in order to promote energy performance levels. The capacitance depends on the porous texture really accessible by the electrolyte. As it happens, for applications in transportation and in particular for an electric vehicle, it is necessary to have a high energy density in order to limit the on-board weight of the supercapacitor, which imposes having a high mass capacitance.
The potential of a supercapacitor depends mainly on the nature of the electrolyte used, which may be organic or aqueous.
There are various possibilities for incorporating the active material into a supercapacitor electrode. Documents U.S. Pat. No. 6,356,432, US-A1-2007/0146967 and U.S. Pat. No. 7,811,337 describe the dispersion of conductive porous carbons in a non-active organic binder and a solvent, then the coating of the paste obtained on the current collector. This method has the drawback of using a binder which makes the system heavy without being active for storing energy.
In the context of an application for an electric vehicle, it is favorable to use, as electrode active material, a carbon monolith in an aqueous electrolyte, in order to maximize the specific energy of this electrode. In order to achieve operation at high powers typically greater than 1 kW/kg, it is necessary for the carbon monolith to be very thin, having a thickness of only a few hundred micrometers and usually less than or equal to 0.5 mm, while being sufficiently robust so as not to be brittle and to not deform at these very small thicknesses.
For the preparation of such a carbon monolith for supercapacitor electrodes, pyrolysis of resorcinol/formaldehyde (RF) gels is usually carried out. The RF resins are in fact particularly advantageous for the preparation of carbon with a high porosity in monolith form, since they are very inexpensive, can be used in water and make it possible to obtain various porosities and densities according to the preparation conditions.
However, since the mixture of resorcinol R and formaldehyde F precursors in water has a very low viscosity, it cannot be coated with a sufficiently small thickness, i.e. typically less than 1 mm, and, instead of such a coating, it is chosen to have the mixture of R and F precursors in a closed mold so as to form a gel after polymerization reaction. In order to limit the adhesion of the mixture to the walls of the mold, it is necessary to provide this mold with a typically fluorinated, non-stick coating, which generates a high production cost.
Another drawback of the existing RF gels for supercapacitor electrodes is that they are chemical gels which are by definition irreversible, since they are obtained by polycondensation of the liquid precursors in the mold. Consequently, once formed, the gel cannot be reused. Furthermore, at high conversion, this gel becomes hydrophobic and precipitates out, which induces mechanical stresses in the material and therefore a greater fragility. Thus, it is necessary to use a method for drying the water present in the gel that is sufficiently mild to prevent fracturing or contraction of the gelled structure, such as supercritical drying (for the formation of an aerogel), lyophilization (for the formation of a cryogel), or very slow drying in a humid chamber (for obtaining a xerogel). The dried gel is then pyrolyzed under nitrogen at high temperature so as to obtain a monolithic porous carbon.
As it happens, one limitation of the current methods is the deformation of the monoliths during pyrolysis, due to the residual stresses when the gel thickness is less than 2 mm. As it happens, in order to obtain carbon electrodes having the abovementioned thickness less than or equal to 0.5 mm, these methods must also comprise a final polishing/rectifying step which has the drawback of being expensive and difficult to implement, and of generating considerable losses of material.
By way of illustration of the prior art presented above for the preparation of monolithic carbons derived from RF gels for supercapacitor electrodes, mention may be made of document U.S. Pat. No. 6,737,445 which teaches the use of a high amount of a cationic, anionic or nonionic surfactant for forming an emulsion in water and polymerizing therein the R and F precursors. An irreversible aqueous chemical gel is obtained which is incapable of being coated with a small thickness, and after drying of this gel under a gas stream and pyrolysis, a mesoporous carbon-based structure, the size of the pores of which corresponds to micelles, is obtained.
Another drawback of this process lies in the mesoporous structure obtained for the carbon which, in the case of a supercapacitor, is unfavorable in comparison with a mainly microporous structure which is preferred for having a high specific energy and a high capacitance. Furthermore, the use of a large amount of surfactant proves to be expensive.
It is also possible to mention, by way of prior art for the preparation of such electrodes, the article “A novel way to maintain resorcinol-formaldehyde porosity during drying: Stabilization of the sol-gel nanostructure using a cationic polyelectrolyte, Mariano M. Bruno et al., 2010”, which discloses a mesoporous monolithic carbon derived from an RF aqueous chemical gel comprising, in addition to a sodium carbonate-based basic catalyst C, a cationic polyelectrolyte P consisting of poly(diallyldimethylammonium chloride) which makes it possible to retain the porosity of the gel following air-drying thereof. The gel is prepared with the molar ratios R:F:C:P=1:2.5:9×10−3:1.6×10−2 and the corresponding concentrations [4M]:[10M]:[0.036M]:[0.064M], by polymerizing R and F from the start in the presence of C and P at 70° C. for 24 hours.
A major drawback of the irreversible chemical gels presented in this article lies in their very low viscosity which makes them totally incapable of being coated with a thickness of less than 2 mm.